The subject invention relates generally to assay of nuclear materials and more particularly to the use of self-interrogation and neutron coincidence counting to determine the quantity of fissile nuclides in samples of uranium hexafluoride. This invention is a result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
The measurement of fissile mass in uranium samples is required for safeguards, accountability, and criticality control of nuclear materials. These materials are found in uranium enrichment plants, fuel fabrication facilities, nuclear reactors, and reprocessing facilities. Two references which summarize active and passive methods for analyzing nuclear materials are: 1. Active Nondestructive Assay of Nuclear Materials by Tsahi Gozani, NUREG/CR-0602, January 1981, and 2. Handbook of Nuclear Safeguards Measurement Methods, by Donald R. Rogers, NUREG/CR-2078 (MLM-2855), September 1983. For some types of nuclear materials, the procedures described therein are inadequate or impractical. Passive signatures are often not available or not sufficiently penetrating to sample all of the material. Moreover, neutron interrogation sources employed to overcome these difficulties are costly and require considerable shielding and maintenance. An example of the latter systems can be found in U.S. Pat. No. 3,636,353 "Method and Apparatus for the Nondestructive Assay of Bulk Nuclear Reactor Fuel Using 1 keV. to 1 MeV. Range Neutrons," issued to Samuel Untermyer on Jan. 18, 1972. Therein, the inventor describes irradiation of the uranium fuel elements to be analyzed by neutrons followed by the detection of emitted neutrons and gamma radiation.
The conventional nondestructive, passive method for measuring .sup.235 U enrichment in UF.sub.6 involves the counting of the 186-keV gamma-ray emissions from the .sup.235 U using NaI or germanium detectors. An ultrasonic measurement of the wall thickness of the container must be made to correct for variations in gamma-ray attenuation in the walls. The measured .sup.235 U enrichment, the net UF.sub.6 weight, and the uranium weight fraction must be combined to yield the desired .sup.235 U mass. A disadvantage of this type of measurement is that it is the enrichment of the outer 5-10 mm of the sample only that is sampled due to strong absorption of the gamma-rays by the bulk of the sample. Therefore, it is necessary to assume that the sample is homogeneous in order to interpret the results. Other disadvantages of the above-described procedure include the fact that a separate measurement of the wall thickness must be made, and that if the sample has been irradiated in a reactor, it may contain sufficient quantities of technetium that increase the gamma-ray background from the sample, thereby altering the accuracy of the measurement.
Gas-phase sampling of the materials under investigation followed by mass spectrometric determination of the .sup.235 U enrichment is often biased by inhomogeneities such as layering in addition to being costly and time-consuming.